Method and apparatus for endpoint detection in a semiconductor wafer etching system

ABSTRACT

A method for focussing a radiant energy beam characterized by the steps of scanning a beam of radiant energy across a test pattern including areas of differing reflectivity, detecting the variance in a reflected portion of the scanned beam and adjusting the beam to minimize the variance. Preferably, the test pattern includes areas of varying widths, e.g. relatively non-reflective areas of varying widths separated by reflective areas of uniform widths. As the beam is scanned perpendicularly across the test pattern it will be highly reflected by the reflective areas and will be partially absorbed by the non-reflective areas. If the beam is wider than a non-reflective area a portion of the beam will be absorbed and a portion of the beam will be reflected, resulting in a greater total reflection than if the beam is narrower than the non-reflective region. In consequence, the intensity of the reflected beam will vary as the energy beam is scanned across the test pattern as it encounters non-reflective areas of varying widths. The amount of variance in the reflected beam is related to the width of the beam and, therefore, the beam can be focussed by adjusting the beam to minimize this variance.

This is a continuation of copending application Ser. No. 07/446,025filed on 12/5/89, now abandoned, which was a divisional of patentapplication Ser. No. 07/221,979 filed on 07/20/88, now U.S. Pat. No.4,953,982.

BACKGROUND OF THE INVENTION

This invention relates generally to front-end integrated circuitprocessing methods and equipment, and more particularly to laserinterferometer end point detection systems.

Integrated circuits are fabricated in bulk on semiconductor wafers.Typically, scribe lines are provided on the surface of a wafer in a gridpattern, such that the individual integrated circuits or "chips" can beeasily separated from each other. After the wafer is fully processed itcan be broken or cut along the scribe lines to separate the chips forpackaging.

The semiconductor wafer is repetitively subjected to a number ofprocesses during the integrated circuit fabrication procedure such asmasking, etching, layer formation, and doping. The present invention isprimarily concerned with the etching process, i.e. the removal of layersof materials from the surface of semiconductor wafers.

As the density and complexity of integrated circuits increase the sizesof the various features within the integrated circuits necessarydecrease. This evolution towards small feature sizes requires a highlevel of control over the etching process. For example, the etchingprocess should be highly anisotropic so as to provide an edge profilewith a high aspect ratio. Also, the etching process should be verycontrollable so that the etching process can be predictably stoppedafter the layer has been etched through.

Determining when a layer has been etched through can be an open-loopprocess, i.e. the etching process can be allowed to progress for apredetermined period of time with the hope that the layer will be etchedthrough without too much over-etching. However, since wafers do notalways etch at the same rate, there is a tendency to run the etchingprocess long enough to etch through the slowest etching wafers,resulting in over-etching of the faster etching wafers. As feature sizesbecome smaller, this results in an unacceptably high defect rate and,thus, this open-loop method is not used on state of the art etchingequipment.

Most modern etching equipment make some provisions for endpointdetection, i.e. detection of etch-through in a desired layer. Oneapproach which is useful with semitransparent layers such as silicondioxide (SiO₂) is to use the principles of laser interferometry. Withlaser beam interferometry, a laser beam is directed at the layer beingetched and a reflected portion of the beam is detected by an appropriatephotodetector. Since the etching layer is semi-transparent to thefrequency of laser light being used, some of the incident beam will bereflected from the top surface of the layer and some of the beam will bereflected from the bottom surface of layer. These two reflections willeither constructively or destructively interfere with each other,creating a characteristic sinusoidal etching curve as the layer isetched away. When the etching curve flattens out, the layer has beenetched through and endpoint has been detected.

One such laser interferometer system is described in U.S. Pat. No.4,618,262 of Maydan et al, which is assigned in common with the presentinvention to Applied Materials, Inc. of Santa Clara, California. In theMaydan et al, patent, a process is described which includes scanning alaser beam across scribe lines on a wafer being etched, and monitoringthe resultant interference pattern. Alternatively, Maydan et al, teach aprocess including the scanning of a laser beam across a wafer to find ascribe line, locking the laser beam on to the scribe line, and thenmonitoring the resulting interference pattern. In both processes, thesize of the laser beam spot is comparable to the width of the scribelines. In other laser interferometer end point detection systems, thelaser spot is much larger than the width of the scribe lines.

While laser interferometer systems such as the one taught by Maydan etal, perform admirably, they have been found to be sensitive to variousforms of noise. For example, it has been found that a great deal ofnoise is generated at the detector when the laser beam scans over a stepor transition on the wafer, and that noise is at a minimum on surfaceshaving uniform film thicknesses. Such transitions are commonly producedby photomasking processes where a patterned layer of photoresist isdeveloped on top of a layer to be etched. In consequence, when the laserbeam spot is comparable to or larger than the width of the scribe linesit is inevitable that a certain amount of noise will be generated as thebeam traverses the scribe lines, since a portion of the beam will alwaysbe on a transition.

The noise amplitude obtained while traversing a transition can besignificantly higher than the amplitude of the etching curves of theflat surfaces. In the past, the laser beam might lock onto such atransition due to this high noise level, resulting in the monitoring ofthe photoresist or a combination of photoresist and silicon dioxide. Theusefulness of a smaller beam size with an adequate depth of field toalleviate this problem was heretofore unrecognized.

Other factors which can generate noise in a laser interferometer systeminclude air currents, heat shimmer, and machine vibrations. While thesefactors can never be eliminated entirely, efforts should be made tominimize each of these factors in a precision laser interferometerendpoint detection system.

It is also important that laser interferometer endpoint detectionsystems recognize the flattening of the characteristic etching curvequickly and accurately. In Maydan et al. a computer is used to recognizethe flattening of the characteristic etching curve by monitoring itsslope. However, it always desirable to develop improved methods whichrecognize the endpoint condition more quickly and more repeatably runafter run.

SUMMARY OF THE INVENTION

This invention automatically and reliably detects the endpoint of asemiconductor wafer etching operation. The method of the inventionincludes the repetitive scanning of a wafer with a laser beam along asingle scan path and the detection of a portion of the laser beam whichis reflected off of the wafer. The reflected beam is analyzed todetermine a preferred parking spot within a preferred flat area of thewafer. Once the preferred parking spot has been determined, the beam ismoved to the spot and the detector is used to monitor the characteristicetching curve of the thin film. Once the etching curve flattens off, thepreferred flat area is considered to be etched through, and an endpointsignal is generated.

The invention includes three major process, a first of which involvesthe selection of the preferred parking spot. In this first process,pairs of scans are compared to develop a quality factors Q correspondingto the central location of the widest flat area and highest etch ratefound in any two scans. After comparing a number of scan pairs, themaximum value Q_(max) is used to select the preferred parking spot. In asecond process, the actual etching curve is compared to a projectedetching curve after each maxima is detected. When the actual etchingcurve and the projected etching curve deviate substantially, endpoint isdeclared by the system. In a third process, a laser beam is focussed byusing a novel test wafer and focussing method. The focussing methodincludes scanning the laser beam across a test pattern having lines ofvarying widths; detecting the variance in the reflected portion of thebeam; and adjusting the focus of the beam to minimize the variance.

The apparatus of the present invention includes a beam forming assembly,a scanning assembly, a detection assembly, an environmental isolationassembly, and a controller. The environmental isolation assemblyisolates the beam forming assembly from the reaction chamber of theetcher, thereby reducing aberrations due to air currents, heat shimmer,etc. The beam forming assembly includes an optical isolation assembly toprevent reflection of the laser beam back into the laser, a beamexpanding assembly, and a beam focussing assembly for focussing theexpanded beam onto the semiconductor wafer. The controller is responsiveto an output of the detection assembly, and is operative to carry outthe aforementioned processes of the present invention.

Due to the improved optics of the apparatus and the focussing method ofthis invention, the spot size of the laser beam has been reducedconsiderably over the previous state of the art. In the past, thesmallest laser spot size having adequate depth of field wasapproximately 100 micrometers, while the spot size of this invention canbe less than 40 micrometers. This small spot size permits a preferredparking spot to be chosen within a preferred flat area having a width of70 micrometers or more. Since the laser beam spot is completely withinthe preferred flat area, noise associated with reflection from thetransition boundaries of the flat area have been eliminated.

The method of this invention for detecting endpoint is quick, accurate,and repeatable. Typically, endpoint can be detected within a smallfraction of the etching cycle, permitting the etching system to bequickly shut down or to be run for a predetermined time after endpointdetection.

These and other advantages of the present invention will become apparentto those skilled in the art after reading the following descriptions andstudying the various figures of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a scribed semiconductor wafer illustratinga typical laser beam scanning path employed by the method of the presentinvention.

FIG. 2a is a cross-sectional view of a partially processed semiconductorwafer illustrating a laser beam located at various spots along thesurface of the wafer.

FIG. 2b is a cross-sectional view of a partially processed semiconductorwafer with a laser beam parked at a spot on the surface of the wafer.

FIG. 3a is a graph of reflected interference pattern waveforms from twodifferent spots on the semiconductor wafer of FIG. 2a.

FIG. 3b is a graph of reflected interference pattern waveforms as alaser beam is scanned along a wafer as shown in FIG. 1 and then parkedon a spot as shown in FIG. 2b.

FIG. 4 is a graph of seven data-points taken in two successive scans ofa semiconductor wafer.

FIGS. 5a and 5b are graphs used to illustrate a preferred method forfinding an optimal spot to park a laser beam.

FIGS. 6a, 6b, and 6c are portions of an waveform used to illustrate apreferred method for endpoint detection.

FIGS. 7a and 7b illustrate the optical path of a laser beam through anendpoint detection apparatus in accordance with the present invention.

FIG. 8 is a top plan view of the endpoint detection apparatusillustrated in FIG. 7a.

FIG. 9 is a top plan view of a test pattern formed on a semiconductorwafer which is used to help focus a laser beam.

FIGS. 10a and 10b illustrate an unfocussed and a focussed laser beam,respectively, impinging upon different portions of the test pattern ofFIG. 9.

FIGS. 11a and 11b are graphs of the relative intensities of thereflected laser beams from an unfocussed and a focussed laser beam,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The Basic Method

With reference to the top plan view of FIG. 1, a semiconductor wafer 10is provided with a number of scribe lines 12 which are laid out in agrid pattern on the top surface 14 of the wafer. Individual integratedcircuits are typically batch processed on the wafer 10 within chiplocations 16. After the wafer has been fully processed, the wafer is cutor broken along the scribe lines 12 to separate the individualintegrated circuits for packaging.

The present invention scans a laser beam spot 18 along a path 20 to alocation 18'. This scanning operation is accomplished automaticallyunder computer control, and is highly repeatable such that multiplescans can be accomplished along a substantially identical scan path 20.Since the path 20 is preferably several centimeters long, it will beappreciated that the laser beam spot 18 will cross at least severalscribe lines 12 and chip locations 16. If, for some reason, the path 20does not cross a scribe line, the apparatus can be manually adjustedalong an axis 22 such that the beam path 20 does cross a scribe line 12.The adjusted path would still, however, be parallel to the path 20depicted in FIG. 1.

With reference to FIG. 2a, laser beam spot 18 is produced by an incidentlaser beam 24 having an axis 26 which is slightly non-perpendicular tothe plane 28 of the top surface 14 of the wafer 10. For example, axis 26of the incident beam 24 can be approximately 91° from the plane 28. Thiswill cause a reflected beam 28 to have an axis 30 which is approximately89° from the plane 28. Thus, in this example, there is an approximately2° separation between the axes of incident beam 24 and reflected beam28. This minimizes the possibility of reflected light adverselyeffecting the desired constant output of the laser beam source.

The cross-sections of FIGS. 2a and 2b illustrate typical layers of apartially processed semiconductor wafer 10. These layers include thesemiconductor substrate, a silicon dioxide (SiO₂) layer 34, and aphotoresist layer 36. As seen in FIG. 2a, as the incident beam 24 isscanned across the top surface 14 of the wafer 10 it will alternatelyproduce a beam spot on the photoresist layer 36, then the silicondioxide layer 34, and then on the photoresist layer 36 again. This isindicated by the incident beam positions 24, 24', and 24". This portionof the process is referred to herein as the "scanning mode".

It should be noted that the width w of the beam spot 18 is considerablysmaller than the width W of an opening, such as scribe line S, in thephotoresist layer 36. In the present embodiment of this invention, thebeam spot is approximately 35 micrometers, while the typical width of ascribe line S is on the order of 80 micrometers. As will be discussed ingreater detail subsequently, since the beam spot 18 is scanned alongsurface 14 in steps in the order of approximately 5 micrometers, it ispossible to obtain a number of data samples from a flat surface 38within opening S. This is important in that a great deal of noise isgenerated when the beam spot falls on steps or transitions such astransitions 40 between the photoresist layer 36 and the silicon dioxidelayer 34.

As noted previously, the beam spot 18 is preferably scanned along path20 several times, e.g. three or four times. A process, which will bediscussed in detail subsequently. is then used to determine a preferredparking spot within a preferred flat area on the wafer 10. For example,and with reference to FIG. 2b, we will assume that the preferred parkingspot P is determined to be within flat area 38. The beam spot is thenmoved to parking spot P and is left there for the remainder of theetching process. This mode of operation is known herein as the "parkingmode".

A bit of the theory of this invention will be discussed with referenceto FIGS. 3a and 3b. In FIG. 3a, a characteristic photoresist etchingcurve is shown at 42, and a characteristic silicon dioxide etching curveis shown at 44. These curves represent the intensity of the reflectedbeam 28 as the etching process progresses, and are substantiallysinusoidal. The curve 42 is typical of the intensity of the reflectedbeam if the incident beam is parked on photoresist 36, and the curve 44is typical of the intensity of the reflected beam if the incident beamis parked on silicon dioxide 34.

The basic theories of thin film laser beam interferometry are well knownto those skilled in the art. For example, a description of the theoriescan be found in the aforementioned Maydan et al, patent, which isincorporated herein by reference. Briefly, when a beam of light impingeson a semi-transparent film such as silicon dioxide, a portion of theincident light will be reflected from the top surface of the film and aportion of the incident light will be reflected from the bottom surfaceof the film. Since the film has a finite thickness, the two reflectionswill either constructively or destructively interfere with each other.As the layer is etched, its thickness is changed, thereby cycling theintensity of the reflected beam through constructive and destructiveinterference patterns to create the sinusoidal patterns shown.

It should be noted that the photoresist etching curve 42 is of greatermagnitude and lower frequency than the silicon dioxide etching curve 44.The higher amplitude is due to the higher reflectivity of photoresist,and the lower frequency is due to the fact that photoresist etches muchmore slowly than silicon dioxide. Because the characteristic etchingcurves of photoresist and silicon dioxide are so different, it is notdifficult for a system to distinguish between them as a laser beam isscanned across the surface of the wafer.

In FIG. 3b, the intensity of the reflected beam 28 is plotted againsttime as the beam is first scanned across the wafer surface 14 as shownin FIGS. 1 and 2a, and then parked at a preferred spot P as shown inFIG. 2b. More specifically, the process is in the scanning mode from atime 0<=t<=t₁, and it is in the parking mode from time t₁ <t<=t₂. Attime t₂ endpoint is detected by the flattening of the actual etchingcurve 46, indicating that all of the silicon dioxide at preferred spot Phas been removed.

The actual etching curve 46 of FIG. 3b has a spiked appearance duringthe scanning mode because the reflected beam varies greatly in intensityas the incident beam is scanned across different surfaces and layers ofthe wafer 10. During the parking mode, the laser beam is parked on thelayer being monitored, in this case silicon dioxide, and, as such, thecurve takes on the characteristic etching curve of the silicon dioxidelayer. As mentioned previously, the actual etching curve 46 flattens outafter the silicon dioxide layer is etched away, indicating endpointdetection.

The preferred processes for picking a preferred parking spot anddetecting endpoint from the actual etching curve will be discussedbelow. However, it should be understood that these methods are notlimitations on the present invention but, rather, teach the best modecurrently known for practicing the invention.

Preferred Parking Spot Detection Method

To ensure optimal accuracy of the interferometer process, the laser beamshould be parked on a widest, flattest surface of the SiO₂ that can bedetected along the scan path 20. Since the scribe lines 12 tend to beorders of magnitude greater in width than integrated circuit features,the optimal surface will most often be found within the boundaries of ascribe line 20.

For the purposes of discussion, it will be assumed that the beam spot 18is stepped N times per scan along the scan path 20, and the intensityvalues of the reflected beam 28 are stored as data values in a digitaldatabase. These data values are stored as vectors associated with eachscan, i.e. the first scan produces a vector S1 of N data values, thesecond scan produces a vector S2 of N data values, etc. As will bediscussed in detail below, these vectors can be mathematicallymanipulated to determine the preferred parking location for the laserbeam spot 18.

With reference to FIG. 4, an arbitrary data value n is chosen as thecenter of a set of 2a+1 data points ranging from n-a to n+a, where inthis example a=3. The seven data values of vector S1 which are centeredat n are then compared against the corresponding seven data values ofvector S2 which are centered at n to determine the minimum absolutedifference 48 between the two. This minimum difference 48 will bedesignated S1S2_(min), and is always a positive value. Next, 2b+1 datavalues of vector S1, which are again centered at n, are compared to findthe maximum absolute difference 50 between any two within the scan. Inthis example b is chosen to be 2. This maximum difference will bedesignated S1_(max), and is also always a positive value. Similarly, thefive data values of vector S2 which are centered at n are compared findthe maximum absolute difference 52 to arrive at a positive valueS2_(max). A relative value R is then calculated as follows:

    R=S1S2.sub.min -[S1.sub.max +S2.sub.max ]

where if R<0 then R=0

The value of n is then incremented by one, and the next relative value Ris calculated as described above. The sequentially derived values for Rare stored in a vector R_(1:2).

For N data points, the first value for n will typically be a+1, whilethe last value for n will be N-(a+1). It therefore follows that forvectors of N data values, there will be N-(a+2) values in the vectorR_(1:2). These values are then compared to determine a Quality FactorQ_(1:2), which is defined as the maximum data value within vectorR_(1:2).

The process described above is then repeated by comparing the Scan Twovector S2 against the Scan Three vector S3 to determine a Quality FactorQ_(2:3), Scan Three vector S3 against Scan Four vector S4 to determine aQuality factor Q_(3:4), etc. As a final step, all of the quality factorsare compared to find the maximum quality factor Q_(max). It is at thedata point corresponding to Q_(max) that the beam is parked. Thisprocess is summarized below in Table 1.

                  TABLE 1                                                         ______________________________________                                         ##STR1##                                                                     ______________________________________                                    

The theory behind this process will be discussed with reference to FIGS.5a and 5b. First, we will ignore the effect of different distancesbetween data points between pairs of scans. In FIG. 5a, both the ScanOne and the Scan Two data values form straight, horizontal lines. Inconsequence, for any seven contiguous points centered at a data value n,the value of S1S2_(min) is a constant. Also, S1_(max) and S2_(max) areequal to zero, since the data values are horizontally aligned. In FIG.5b the seven contiguous points centered at data value n are also allequidistant between Scan One and Scan Two such that the value S1S2_(min)is also a constant. However, in FIG. 5b the values S1_(max) and S2_(max)are both greater than zero. Therefore the value of R for the FIG. 5aplot will be greater than the value of R for the FIG. 5b plot by theamount [S1_(max) +S2_(max) ] of the FIG. 5b plot. The S1_(max) andS2_(max) values represent a lack of flatness in the area centered atdata point n. Since FIG. 5a has a larger R value than FIG. 5b, the laserbeam would rather park on the spot represented by FIG. 5a than the spotrepresented by FIG. 5b, i.e. on the flatter spot.

The effect of S1S2_(min) is to center the preferred spot within thepreferred flat area. This is because S1S2_(min) decreases as the datapoint approaches a transition. In consequence, S1S2_(min) tends to be amaximum at the center of any particular flat area. It is therefore clearthat the formula: R=S1S2_(min) -[S1_(max) +S2_(max) ] is assigning avalue to R which reflects the relative desirability of the spot, bothwith respect to it being in the largest, flattest area available andwith respect to it being centered within that area.

It should be noted that there must be a phase difference betweenadjacent scans to ensure that the two scans do not coincide. This can beaccomplished by making the scanning frequency higher (e.g. 3-4 timeshigher) than the frequency of the characteristic etching curve of thesilicon dioxide.

Endpoint Detection Method

As mentioned previously with reference to FIG. 3b, once the laser beamhas been parked on a spot of oxide that is being etched, the reflectedinterference pattern will typically take a sinusoidal shape. When theoxide has been etched through, the sinusoid will flatten, indicating endpoint detection. At that point in time, the etching process is usuallyterminated.

It is not necessarily disastrous or even undesirable to over-etch, i.e.continue to etch after the oxide has been completely etched through. Infact, many processes deliberately over-etch to ensure complete removalof film being etched in areas of the wafer that are not being monitored.What is crucial is the repeatability of the etch process: the sameamount of etching must predictably occur cycle after cycle. Accurateendpoint detection is therefore important so that the desired amount ofover-etching (if any) can be accomplished.

With reference to FIGS. 6a and 6b, the actual etching curve 46 iscontinuously monitored by the system to determine its Peak-To-Peak (PTP)value. This PTP value can be updated every 1/2 cycle of the actualetching curve. A calculation is then made to calculate h, which is, inthis example, 20% of the PTP value. As seen in FIG. 6b, the value hdefines a box 54 having corners at points A, A' on the actual etchingcurve which encloses the top 20% of the curve. The centerline of the boxis designated by line C_(L), and the width of the box is designated as2W.

With reference to FIG. 6c, when the system detects that the actualetching curve 46 interference pattern has reached point A, i.e. thecurve has entered the area of the box 54, the intensity value of A isstored within memory. As the oxide continues to etch, a series ofintensity values (represented here by points B, C, . . . , G) up to themaximum value I_(max) on the intensity pattern are also stored withinmemory. These values are then reflected around the centerline C_(L) tocreate pseudo-values A', B', . . . , G' which define a projected curvesegment 56. A threshold line 58 is defined as being halfway between theI_(max) value and the projected curve 56. In other words, the thresholdline will have points at (I_(max) -G')/2; (I_(max) -F')/2; (I_(max)-E')/2; etc. The actual etching curve 46 after the centerline C_(L) arethen detected, stored, and compared against the threshold line 58. Ifall of the data values of the actual etching curve 46 centerline C_(L)are above the threshold line 58 then endpoint has been detected.Otherwise, the surface is still etching, and the above process isrepeated until endpoint is detected. It will be noted that this methodwill find the endpoint within h % of the PTP value of the characteristicetching curve, which can be arbitrarily small depending upon the desiredendpoint detection sensitivity.

Endpoint Detection System Apparatus

The apparatus of the present invention will be discussed with referenceto FIGS. 7a, 7b, and 8. In FIG. 7a, an endpoint detection system 60includes a laser 62, an optical isolation assembly 64, and expanderassembly 66, a mirror 68, a focussing assembly 70, a window assembly 72,a collector assebly 74, a detector 76, and a controller 78. The system60 is enclosed within an enclosure 79 to protect the delicate optics andto minimize noise from such factors as air currents, etc.

The laser 62 is preferably a commercially available, polarizedhelium-neon (HeNe) gas laser. The beam 80 of the laser 62 is directedtowards the optical isolation assembly 64, which minimizes the amount oflight reflected back to the laser 62. Such reflected back light isproblematic in that it can cause intensity drift of the laser beam 80.More specifically, the optical isolation assembly includes a polarizingbeam splitter 82, and a 1/4 wave plate 84. The axis of polarization ofthe beam splitter 82 is aligned with the axis of polarization of thepolarized laser 62, and the 1/4 wave plate 84 circularly polarizes thebeam 80. The combination of these elements reduces back-reflection oflight to the laser 62 considerably because: 1) the back reflected lightwould have to be circularly polarized in phase with the 1/4 wave platepolarization and 2) only that portion of the in-phase back-reflectedcircularly polarized light which aligns with the axis of polarization ofthe beam splitter 82 will make it back to laser 62. This is generally aninsignificant amount. An optional detector 86 can be used to monitor theoutput of laser 62.

Expander assembly 66 includes a pair of lenses 88 and 90 having the samefocal point 92. This arrangement of lenses results in a collimated beam91. This beam is reflected from mirror 68 at substantially right anglesand through focussing assembly 90 and window assembly 72 to the wafer 10(see FIG. 7b), thereby forming incident beam 24. A reflected beam 28 isreflected from the surface of wafer 10 and back through the windowassembly 72, focussing assembly 70, past the edge of the mirror 68, intothe collector assembly 74, and onto the detector 76. The mirror 68 ispreferably a front-silvered mirror of minimal reflective loss.

The window assembly comprises a pair of spaced-apart quartz window panes96. The space 98 between the window panes 96 provides thermal isolationbetween the optics side 100 and the wafer side 102 of the windowassembly. This is an important feature, because the wafer side 102 facesthe etcher's reaction chamber and, thus, is exposed to hightemperatures.

The collector assembly 74 includes a collecting lens 104 and a filter106. The lens helps to focus the reflected beam 28 on the detector 76,while the filter removes undesired frequencies of light generated byplasma discharge, ambient lighting, etc. The detector 76 can be any of avariety of photodetectors sensitive to light in the frequency range oflaser 62.

The output of the detector 76 is input into controller 78, which isbasically a dedicated microcomputer system. The controller 78 providesoutputs to control the stage movements, the operation of the laser, etc.and has an output indicating endpoint detection. The endpoint detectionsignal generated by controller 78 can be used to automatically shut downthe etching process, or to alert an operator to the endpoint condition.

Referring now to FIG. 8, a top plan view of the endpoint detectionsystem 60 shows a laser head 108, an adaptor 110, an optics housing 112,a slidable plate 114, a stage 115, a detector housing 116, and a base118. The detector housing 116 is rigidly attached to the optics housing112 and to the stage 115. A flange 120 of adaptor 110 couples theadaptor to a flange 122 of optics housing 112. A quartz window 96 can beseen in phantom beneath the optics housing 112 and plate 114.

A stepper motor 124 mounted on plate 114 has a lead screw 125 which candrive the stage 115 and thus the optical housing 112 and detectorhousing 116 back and forth as indicated by bi-directional arrow 126.This movement of the optical housing 112 causes the scanning of thelaser beam spot 18 along the path 20. The plate 114 and everythingcarried by the plate 114 can be moved back and forth as indicated bybi-directional arrow 128 after the loosening of thumb screws 130 whichclamp along an edge of plate 114. This manual adjustment corresponds tothe adjustment of the beam path along axis 22 of FIG. 1.

The beam spot can be focussed and aligned by adjusting three screws 132provided at three corners of the base 118. These screws press againstplate 114 to create a conventional three-point adjustment arrangement.With these three screws 132, the pitch, roll, and focus of the beam canbe adjusted, as will be apparent to those skilled in the art. The fourthcorner of plate 114 provides a pivot point for the plate.

Beam Focussing Method

A preferred method for focussing the beam spot will be discussed withreference to FIGS. 9-11. In FIG. 9, a pattern 134 is produced on asurface 136. Alternatively, grooves corresponding to the pattern 134could be cut into a surface 136. In either case, a pattern ofalternating relatively reflective regions 138 and relativelynon-reflecting regions 140 are provided on a planar surface. In thisexample, the non-reflecting regions 140 are of varying width, while thereflecting regions 138 are of substantially constant width. For example,the center region 140 could be 34 micrometers wide, the two regions 140flanking the center region could be 36 micrometers wide, the next twoflanking regions 140 could be 38 micrometers wide, etc.

With reference to FIG. 10a, an unfocussed beam 142 impinging on a widenon-reflecting region 140'a is substantially completely absorbed.However, when the unfocussed beam 142 impinges on a narrownon-reflecting region 140"a, a portion of the incident beam will bereflected at 142'. When the intensity of the reflected beams forunfocussed incident beams being scanned across the test pattern 134 aremonitored, a waveform such as that shown at 11a is developed.

In FIG. 10b, a focussed beam 142 will be absorbed whether it falls on awide region 140'b or a narrow region 140"b. Therefore, there will belittle or no reflected beam for a focussed incident beam. A focussedbeam will therefore produce a wave pattern such as that shown at FIG.11b.

The method for focussing the beam spot involves scanning the wafer in adirection perpendicular to the pattern 134, and monitoring the intensityof the reflected beam. The beam adjustment screws 132 (see FIG. 8) arethen adjusted to minimize the variance (and therefor the averageintensity) of the reflected beam. The beam spot will be focussed at thepoint of minimum detected variance.

Of course, many other test patterns can be used other than the oneshown. Furthermore, the pattern could have reflective regions of varyingwidth, where the object is to minimize the variance by maximizing theaverage intensity.

While this invention has been described with reference to severalpreferred embodiments, various alterations and permutations of theinvention will no doubt become apparent to those skilled in the art upona reading of the preceding descriptions and a study of the variousfigures of the drawing. For example, while this invention has discussedprimarily the etching of oxide beneath a photoresist mask, a variety ofother etched and masking materials can be used. For example, othersuitable materials to be monitored for endpoint detection includepolysilicon, silicon nitride, and BPSG. It is therefore intended thatthe scope of the present invention be determined by the followingappended claims.

What is claimed is:
 1. A method of focussing a beam of radiant energycomprising:scanning a beam of radiant energy across a test patterncomprising areas of differing reflectivity, said test pattern includinga first plurality of areas of a first reflectivity alternated with asecond plurality of areas of a second reflectivity, said first pluralityof areas being of variable width; detecting an intensity variance in areflected portion of said beam as it is scanned across said firstplurality of areas; and adjusting the focus of said beam to minimizesaid intensity variance of said reflected portion between successiveareas of said first plurality of areas.
 2. A method as claimed in claim1 wherein said first plurality of areas and said second plurality ofareas define separate planes.
 3. A method as claimed in claim 1 whereinsaid second plurality of areas are of variable width.
 4. A method forfocussing a beam of radiant energy comprising:(a) varying the focus of abeam of radiant energy; (b) scanning said beam of radiant energy acrossa test pattern comprising areas of differing reflectivity; (c) analyzinga reflected portion of said scanned beam against a pre-existing criteriato determine the relative focus of said beam; and (d) repeating steps a,b and c to improve the focus of said beam.
 5. A method as claimed inclaim 4 wherein said pre-existing criteria includes criteria derivedfrom a previous scan of said beam across said test pattern.
 6. A methodas claimed in claim 4 wherein said test pattern includes a firstplurality of areas of a first reflectivity alternated with a secondplurality of areas of a second reflectivity.
 7. A method as claimed inclaim 6 wherein said first plurality of areas are of variable width. 8.A method as claimed in claim 7 wherein said second plurality of areasare of variable width.
 9. An apparatus for focussing a beam of radiantenergy comprising:a test pattern comprising areas of differingreflectivity; source means for generating a beam of radiant energy;means for varying the focus of said beam; means for scanning said beamacross a surface of said test pattern; detection means for detecting aportion of said beam reflected from said test pattern; and analysismeans for analyzing the reflected portion of said beam against apre-existing criteria to determine the relative focus of said beam. 10.An apparatus as claimed in claim 9 wherein said pre-existing criteriaincludes criteria derived from a previous scan of said test beam acrosssaid test pattern.
 11. An apparatus as claimed in claim 9 wherein saidtest pattern includes a first plurality of areas of a first reflectivityalternated with a second plurality of areas of a second reflectivity.12. An apparatus as claimed in claim 11 wherein said first plurality ofareas are of variable width.
 13. An apparatus as claimed in claim 12wherein said second plurality of areas are of variable width.
 14. Atarget useful for focussing a beam of radiant energy comprising:a bodyhaving a test surface provided with a first plurality of areas of afirst reflectivity alternating with a second plurality of areas of asecond reflectivity, said first plurality of areas being of variablewidth ranging from a width greater than the width of a focussed beamspot to a width less than the width of a focussed beam spot, said firstplurality of areas being varied in width in a regular pattern.
 15. Atarget as recited in claim 14 wherein said second plurality of areas areof substantially constant width.
 16. A target as recited in claim 14wherein said second plurality of areas are of variable width.
 17. Atarget as recited in claim 14 wherein said first plurality of areas arein a different plane of said surface than said second plurality ofareas.